The adenovirus host-cell reactivation assay, in which adenovirus 2 or 5 is treated with physical or chemical agents, and subsequently assayed for biological activity on monolayers of human fibroblasts, either capable of repairing ultraviolet-damaged DNA or not, was used to study the action of several compounds on DNA. (Agents that make DNA damage repairable by ultraviolet repair mechanisms reduce the plaquing ability of adenovirus more rapidly if repair-deficient (xeroderma pigmentosum) fibroblasts are used as viral hosts, than if normal ones are used.) Chlorpromazine, a commonly used antipsychotic phenothiazine drug which sensitizes patients to sunlight, was found to cause damage to DNA when virus-chlorpromazine solutions were exposed to 330-390nm light. About 70% of this damage was repairable. Further, single-strand breaks were produced in the DNA by this treatment. A similar result was obtained when 6-hydroxybenz(a) pyrene plus small amounts of copper were used to inactivate the virus. Preliminary results with the suspect ultimate carcinogen of benz(a)pyrene, 7,8 diol-9, 10 epoxy benz(a)pyrene, showed that about 90% of the DNA damage it makes is subject to DNA repair mechanisms.